In large area electronic circuits including active-matrix liquid crystal display (AM-LCD) technology, active-matrix organic light emitting displays (AM-OLED) and x-ray imagers, thin film transistors are used to form the switching circuits. For example, in an AM-LCD display, an array of TFTs are used as the active “switches”, each TFT controls one of the pixels of the displays. As shown in FIG. 1, an LCD display (100) consists of a two dimensional array of pixels having data lines (D1-DM) and gate selection lines (S1-SN). A TFT is inserted between the gate selection line and the data line as an active switch to control the ON/OFF states of the pixel defined by the selection line and data line. The liquid crystal pixel has an equivalent capacitance Cp. One end of Cp is connected to the drain of the TFT whereas the other end of it is connected to a ground line (in this case the next selection line).
During operation, the TFT behaves like a variable resistor with low ON state resistance RON and high OFF state resistance ROFF. The rise time of the pixel voltage when the TFT is turned on should be short while the hold time or charge retention time should be long. The short rise time is achieved by the low RON value and the long retention time is achieved by the large ROFF. In order to achieve long enough charge retention time, in many electronic displays, a storage capacitor CS with capacitance value substantially larger than that of the liquid crystal cell (Cp), is usually inserted into each pixel to minimize the unwanted fast discharging. As the main components in the backplanes of AM-LCDs, the device performance of TFTs often determines the quality of the displays, such as response times and contrast ratios. Therefore, the search for high performance TFTs has never stopped.
FIG. 2 shows an enlarged view of a pixel (200) in an AM-LCD display. There is a first substrate (201) with a gate select line (202) and gate electrode (203) deposited and patterned. The gate electrode (203) and gate select line (202) are connected electrically. A layer of gate insulator (204) is deposited to cover the gate metal (203) with a drain electrode (205) and a source electrode (206) formed on top of it. An active channel layer (207) is deposited to cover part of the gate insulator layer (204) and to overlap partly the source and drain electrodes. The source electrode (206) is connected to a transparent pixel electrode (208) which is conductive and optically transparent. The drain electrode (205) is connected to a data line (209). To form the LCD units, a second substrate (not shown) with a conductive and optically transparent layer is brought over the first substrate (201), with the conductive layer on the second substrate facing the first substrate where the TFT (200) and the transparent pixel electrodes (208) are deposited. Liquid crystal is introduced into the gap space between the firs substrate (201) and the second substrate.
Two most important parameters to the operation of a TFT are the response time and ON/OFF current ratio (ION/IOFF). The response time is mainly determined by the RC time constant for charging the pixel, which is equal to RON×CS (where RON is the ON state resistance of the TFT and CS is the capacitance of the storage capacitor). Value of RON is determined by charge carrier mobility in the active channel layer (207) and the dimensions of the active channel of the TFTs. For TFTs with fixed channel dimensions, RON is inversely proportional to the charge carrier mobility in the active channel layer (207). In TFTs with high charge carrier mobility, values of RON decrease and the unwanted joule heating in the TFTs during operation will be reduced. In addition, when active channel layer with higher charge carrier mobility is used, the dimensions of TFTs may be reduced while the same RON×CS value is maintained. When the dimensions of the TFTs are reduced, the area available for the transparent pixel electrode (208) will increase. As a direct result, the pixel aperture will increase to allow more light to pass through and the brightness of the displays will be increased. Therefore, it is advantageous to develop TFTs having active channel layer with high charge carrier mobilities. Furthermore, due to the reduction in RON values with the increase in charge carrier mobilities of the active channel layer, data lines or selection lines having higher resistance (for example, due to the increase of the dimensions of the displays) can still be adopted to yield small enough RON×CS values for normal operation. The advantages of having TFTs with high channel layer mobility thus include: [1] lower joule heating of the transistors (equal to Id2 RON, where Id is the drain current of the TFT), [2] higher switching speed, [3] larger display dimensions and more importantly [4] larger pixel aperture.
Thin Film Transistors (TFTs)
From the above description, TFT is a three terminal device consisting of a gate electrode, a source electrode and a drain electrode separated by a gate insulator. A thin layer of semiconductor deposited between the source and drain electrodes serves as the active channel region. Upon applying gate voltages, charge carriers can be induced into the channel and travel from source to drain under the influence of a lateral electric filed created by drain voltages, giving rise to a drain currents.
The first TFT was fabricated by P. K. Weimer in 1961 by using thin film cadmium sulfide (CdS) as the active channel semiconductor. Over the last few decades, TFT technology has been developed on several materials including cadmium selenide (CdSe, electron mobility ˜10 cm2/V-s), tellurium (Te, mobility value as high as ˜100 cm2/V-s). However, due to its large OFF state current and its poor thermal stability, the above materials were not used in mass productions for electronic products involving TFTs. The most successful TFT at the present time is based on amorphous silicon (a-Si) due to its mature technology and excellent yield in production. However, amorphous silicon has a relatively low charge carrier mobility (˜1 cm2/V-s), therefore, it suffers from relatively low circuit densities (low pixel aperture) and slow switching speeds because substrates are processed at relatively low temperature (<350° C.).
Extensive efforts have also been made in the last few decades to develop silicon (Si)-based TFTs with higher mobility. In order to increase the mobility, polycrystalline thin films have to be used. By switching amorphous Si to polycrystalline silicon (p-Si), TFTs with electron mobility exceeding 100 cm2/V-s can be created. However, this requires either high temperature (>650° C.) crystallization or low temperature (<600° C.) laser annealing process. Because of the high processing temperatures, expensive substrates are needed to produce polycrystalline silicon TFTs for AM-LCDs, thus preventing this technology from being used in low cost production.
Metal Oxide TFTs
From the above description, it is evident that it is highly desirable to develop TFTs having high carrier mobility (as compared to amorphous silicon TFTs) and with low processing temperatures. Recently, there has been some development on a new class of TFTs utilizing metal oxides as the active channel materials. The metal oxides include indium oxide, zinc oxide, tin oxide, gallium oxide and their mixtures. For TFTs with metal oxides as the active channel materials, the advantages include low processing temperatures and higher charge carrier mobility. As mentioned before, by having higher charge carrier mobility values, dimensions of TFTs required for switching of liquid crystal or OLED pixels can be made smaller resulting in higher pixel aperture and shorter switching time. Due to the increase in the charge carrier mobility values, the new metal oxide TFTs are not only suitable for the switching of pixels in the conventional AM-LCDs but also ideal for driving pixels in organic light emitting diode arrays, which often require high operation currents.
Although the charge carrier mobility of the metal oxide TFTs is higher than that of the a-Si TFTs, the values (in the order of several tens of cm2/V-sec) are still low as comparing to that of the monocrystalline silicon and polycrystalline silicon. Hence, even though TFTs made of these metal oxides are suitable for driving of LCDs and OLEDs displays, the resistance between the drain and the source electrodes in the ON state is still considered high and the unwanted joule heating in the active channel layer is sizeable. Furthermore, the charge carrier mobility of the metal oxides are not sufficiently high to form into devices and circuits for generating and manipulating of electrical signals in a frequency range from few tens of megahertz (MHz) to several gigahertz (GHz)—in the microwave range. If TFTs with charge carrier mobility in the order of several hundred cm2/V-sec can be developed, switching circuits and devices with minimum unwanted joule heating may be realized and power consumption for the operation of such circuits may be minimized. In addition, advanced TFTs circuits required for microwave signals may be fabricated on low cost substrates such as glass and plastic over a large area. In this manner, the manufacturing cost for the switching circuits and high frequency circuits may be reduced significantly.